The plunger is a key component in industrial fluid delivery systems requiring precision flow control. Simply, a plunger is a solid ram which is housed within a necked, hollow bore. When fluid contained in the bore must be ejected, the plunger is moved toward the necked end of the bore and forces fluid out the necked end. Because the bore diameter at the necked end can be made very small relative to the plunger diameter, fluid can be ejected from the bore in extremely accurate amounts.
In many industrial applications, the plunger must be both strong and resistant to degradation in severe environments and so ceramics have been explored as candidate plunger materials. Because its thermal expansion coefficient is close to that of many metals, zirconia has been touted as a potential ceramic plunger.
One industrial application of plungers appears to be particularly suited for zirconia ceramics. High pressure injection systems in diesel engines currently require plungers (or "timing plungers") which are strong, chip-resistant and degradation-resistant. Since strength is typically associated with critical flaw size and chips are associated with porosity and large grain size, the zirconia selected for this particular application should have a low porosity and a small average grain size.
Currently, two types of zirconia ceramics have been examined for their utility as timing plungers for the high pressure, diesel injection system. The first is magnesia partially stabilized zirconia ("Mg-PSZ"). Mg-PSZ typically contains at least about 10 w/o magnesia, about 1% to 3% porosity, and often has a grain size of about 50 microns. Because of its large grain size, Mg-PSZ suffers from low strength (i.e., about 550 MPa) and chipping.
The second type of zirconia examined for plunger applications is zirconia which has been partially stabilized by rare earth oxides ("YTZP"). See, for example, Japanese Patent Publication JP-A-58156578 (KoKoku 90046538), which discloses a YTZP zirconia sliding material useful as a plunger which contains at least 50 mol % tetragonal and at most 10 mol % monoclinic zirconia. The commercial embodiments of YTZP typically contain 2.5-3.0 mol % yttria, are much stronger than Mg-PSZ, have a smaller grain size than Mg-PSZ, and have less porosity than Mg-PSZ and so is an attractive candidate for use as a plunger. However, it is known that YTZP's tend to suffer from low temperature degradation ("LTD") at temperatures of 100-300.degree. C., resulting in a severe loss of strength.
Investigators have attempted many solutions to the LTD problem of yttria stabilized zirconia, the most popular routes including either reducing grain size or increasing stabilizer concentration. Some investigators have found that grain size reduction to about 0.2 microns reduces LTD but at the expense of lowering toughness to less than about 4.0 MPa m.sup.1/2. Other investigators have reported no gain in LTD resistance using submicron zirconia. Investigators who have increased the yttria content to about 3.5 mol % have not produced the required LTD resistance, while investigators who have increased the stabilizer content to about 5 mol % have reported increased LTD resistance but at the expense of reducing fracture toughness to less than about 3 MPa m.sup.1/2.
In sum, the prior art has recognized the strength problems associated with plungers made from Mg-PSZ and has sought to replace it with YTZP. However, since the art has also found that:
a) 3 mol % and 3.5 mol % YTZP possess poor LTD resistance, PA1 b) 5 mol % YTZP possesses low toughness, and PA1 c) grain size manipulation produces uncertain benefits and toughness problems, PA1 a) providing a plunger capable of reciprocal axial sliding movement against an axial bore and having a first end, the bore containing the fluid, and PA1 a) advancing the first end of the plunger through the axial bore, thereby displacing the fluid from the bore, PA1 a) abrading the substrate with a sharp edge, the sharp edge consisting essentially of polycrystalline zirconia partially stabilized by between 3.8 mol % and 4.4 mol % yttria. Preferably, the sharp edge has an angle of no more than 45 degrees, preferably less than 30 degrees. PA1 a) providing a densified ceramic consisting essentially of zirconia partially stabilized by between 3.8 mol % and 4.4 mol % yttria, and PA1 b) contacting the ceramic with a liquid comprising water, PA1 a) providing a densified ceramic consisting essentially of zirconia partially stabilized by between 3.8 mol % and 4.4 mol % yttria, and PA1 b) contacting the ceramic with saturated steam having a temperature of at least 100.degree. C. In one preferred embodiment, the saturated steam has a temperature of between 110.degree. C. and 150.degree. C., while in another the saturated steam has a temperature of between 200.degree. C. and 250.degree. C. In some embodiments using saturated steam, the ceramic has a surface which is contaminated with biological material. PA1 a) providing a densified ceramic consisting essentially of zirconia partially stabilized by between 3.8 mol % and 4.4 mol % yttria, and PA1 b) contacting the ceramic with the atmosphere, the atmosphere consisting essentially of an inert gas at a temperature of at least 400.degree. C. (preferably, at least 700.degree. C., but more preferably between 700.degree. C. and 1000.degree. C.). PA1 a) providing a densified ceramic consisting essentially of zirconia partially stabilized by between 3.8 mol % and 4.4 mol % yttria, the ceramic having a surface, and PA1 b) lapping the surface with an aqueous abrasive slurry. PA1 a) providing a densified ceramic consisting essentially of zirconia partially stabilized by between 3.8 mol % and 4.4 mol % yttria, the ceramic having a sliding and/or rolling surface (and is preferably a race or a bearing ball), and PA1 b) providing rolling contact between the sliding surface and a substrate, thereby producing a hertzian stress upon the sliding surface.
the art has provided no guidance as to how to obtain a strong (i.e., greater than 900 MPa), tough (greater than 4.0 MPa m.sup.1/2), LTD resistant material. Therefore, plunger manufacturers have continued to select Mg-PSZ as their preferred material despite its low strength.
Accordingly, there is a need for a zirconia-based material which fulfills the plunger requirements of high LTD resistance, high strength, low porosity and small grain size.